The present invention is directed to aqueous ink jet ink compositions which include latex binder and which exhibit an advantageous combination of good optical density, good print quality and good wet-rub resistance. The present invention is also directed to latex binders for use in aqueous ink jet ink compositions and ink jet printing methods, and to methods of ink jet printing, which methods provide printed mediums having an advantageous combination of good optical density, good print quality and good wet-rub resistance.
Ink jet printing is a well known process and generally comprises ejecting a droplet of an ink composition through a fine nozzle to record images on a surface of a recording medium. Typically, paper recording mediums are employed although other polymer based film recording mediums have also been used. High quality printing and relatively low noise operation have made ink jet printers particularly advantageous.
Dyes and/or pigments are typically used as colorants in ink jet ink compositions. While dyes often provide very good color properties immediately after printing, they are often light sensitive so that printed images tend to fade after time. Dyes can also remain water soluble after printing, whereby printed images smear when contacted with moisture. On the other hand, pigment colorants often exhibit improved light-fastness and water-fastness as compared with dyes. Typically, a pigment is employed in combination with one or more polymeric dispersants in ink jet ink compositions in order to form a stable dispersion of the pigment particles in the ink.
While inks containing pigment colorants generally exhibit improved water-fastness as compared with inks containing water-soluble dyes, it is often desirable to further improve the water-fastness of pigment-containing ink jet ink compositions, particularly for end-use applications where a printed recording medium may be exposed to high moisture and/or humidity conditions. The water-fastness is often measured as wet-rub resistance.
Recently, various latex binders have been employed in attempts to improve the water-fastness of ink jet ink compositions which contain pigment colorants. Oftentimes, however, a latex binder which improves the wet-rub resistance of an ink jet ink composition is not suitable for use in the ink jet print head nozzles and causes printer maintenance problems. Typically, the print head nozzles have diameters of less than about 30 xcexcm and relatively high temperatures are employed to eject the ink compositions from the nozzles. Under the high temperature conditions, many latex binders form a film on the nozzle plate, resulting in clogging of the nozzle and printhead failure, so that extra maintenance of the printer is often required.
Accordingly, pigment-containing ink jet ink compositions which exhibit good printing properties, particularly in terms of print quality and optical density, which have improved wet-rub resistance, and which are suitable for substantially maintenance-free use in conventional ink jet printing systems are desired. Additionally, substantially maintenance-free printing methods which provide printed recording mediums having good print quality, good optical density, and good wet-rub resistance are desired.
It is therefore an object of the present invention to provide improved ink jet ink compositions. It is a more specific object of the invention to provide ink jet ink compositions which exhibit good optical density and good print quality. It is a further object of the invention to provide such ink jet ink compositions which also exhibit improved wet rub resistance. It is a related object of the invention to provide latex binders for use in ink jet ink compositions and, particularly, for improving wet rub resistance of ink jet ink compositions. It is a further object of the invention to provide improved methods for ink jet printing, which methods employ pigment-containing ink jet ink compositions having latex binder therein.
These and additional objects and advantages are provided by the present invention. In a first embodiment, the invention is directed to aqueous ink jet ink compositions which comprise pigment, humectant, dispersant and a latex binder. The latex binder is formed from first monomer having a glass transition temperature Tg greater than about 70xc2x0 C. and comprising styrene, substituted styrene, methyl methacrylate, or a mixture thereof, second monomer having a glass transition temperature Tg less than about 0xc2x0 C. and comprising a C2-C10 alkyl acrylate, charge stabilizing agent, emulsifier and initiator. The latex binder has a glass transition temperature Tg of from about 0xc2x0 C. to about 70xc2x0 C., an average particle size of from about 150 nm to about 350 nm, and a surface tension of from about 40 dynes/cm to about 60 dynes/cm. In a further embodiment, the invention is directed to methods of ink jet printing, which methods comprise ejecting a droplet of an aqueous ink jet ink composition as described through a nozzle and onto a surface of a paper recording medium. These compositions and methods provide an advantageous combination of good optical density, print quality and wet-rub resistance.
The present invention is also directed to latex binders formed by emulsion polymerization and suitable for use in ink jet ink compositions. The latex binders are formed by emulsion polymerization of first monomer having a glass transition temperature Tg greater than about 70xc2x0 C. and comprising styrene, substituted styrene, methyl methacrylate or a mixture thereof, second monomer having a glass transition temperature Tg less than about 0xc2x0 C. and comprising a C2-C10 alkyl acrylate, charge stabilizing agent, emulsifier and initiator. The latex binders have a glass transition temperature Tg of from about 0xc2x0 C. to about 70xc2x0 C., an average particle size from about 150 nm to about 350 nm, and a surface tension of from about 40 dynes/cm to about 60 dynes/cm.
In a further embodiment, the invention is directed to methods of ink jet printing, which methods comprise ejecting a droplet of an aqueous ink jet ink composition through a nozzle and onto a surface of a paper recording medium, and passing the paper recording medium with the aqueous ink jet ink composition on a surface thereof through a fuser system. The fuser system is operated at a temperature greater than about 100xc2x0 C., and the paper recording medium is passed through the fuser system for about 5 to about 100 seconds. The aqueous ink jet composition employed in these methods comprises, by weight, from about 1% to about 20% pigment, from about 5% to about 50% humectant, from about 0.01% to about 10% dispersant, and from about 1% to about 20% of an emulsion polymerized latex binder having a glass transition temperature Tg of from about 0xc2x0 C. to about 70xc2x0 C. and a particle size of from about 150 nm to about 350 nm. These methods provide printed recorded mediums having an advantageous combination of good optical density, good print quality and good wet-rub resistance.
Additional embodiments, objects and advantages of the present invention will be more further apparent in view of the following detailed description.
The aqueous ink jet ink compositions and the ink jet printing methods according to the present invention provide printed recording mediums which exhibit good optical density, good print quality and good wet-rub resistance.
The aqueous ink jet ink compositions comprise pigment, humectant, dispersant and latex binder in an aqueous medium. The aqueous medium may comprise water, preferably distilled and/or deionized water, or may comprise water in combination with one or more water-miscible organic oils or solvents. In a preferred embodiment, the aqueous medium is deionized water.
A wide variety of organic and inorganic pigments are known in the art for use in ink jet printing systems and are suitable for use in the compositions of the present invention, alone or in combination. The pigment dispersion particles must be sufficiently small to permit free flow of the ink through the ink jet printing device, and particularly the ink jet print nozzles, which typically have diameters in the range of from about 10 to about 50 xcexcm, and more typically of about 30 xcexcm or less. The particle size of the pigment should also be selected to maintain pigment dispersion stability in the ink, and it is generally desirable to use smaller sized particles for maximum color strength. Accordingly, pigment dispersion particles having a size in the range of from about 50 xcexcm to about 200 nm, and more preferably less than about 120 nm, are preferred.
Pigments which are suitable for use in the present compositions include, but are not limited to, azo pigments such as condensed and chelate azo pigments; polycyclic pigments such as phthalocyanines, anthraquinones, quinacridones, thioindigoids, isoindolinones, and quinophthalones; nitro pigments; daylight fluorescent pigments; carbonates; chromates; titanium oxides; zinc oxides; iron oxides and carbon black. In one embodiment, the pigment is other than a white pigment, such as titanium dioxide. Preferred pigments employed in the ink composition include carbon black and pigments capable of generating a cyan, magenta and yellow ink. Suitable commercially available pigments include, for example, Pigment Red 81, Pigment Red 122, Pigment Yellow 13, Pigment Yellow 14, Pigment Yellow 17, Pigment Yellow 74, Pigment Yellow 83, Pigment Yellow 128, Pigment Yellow 138, Pigment Orange 5, Pigment Orange 30, Pigment Orange 34, Pigment Blue 15:4 and Pigment Blue 15:3. The pigments may be prepared via conventional techniques.
The ink compositions also include a dispersant, typically for dispersing the pigment therein. The dispersant may be polymeric or nonpolymeric. The term xe2x80x9cpolymeric dispersantxe2x80x9d as used herein, is meant to include homopolymers, copolymers, terpolymers and immiscible and miscible polymer blends. Suitable non-polymeric dispersants include naphthalene sulfonic acid, sodium lignosulfate and glycerol stearate. Numerous polymeric dispersants are known in the art and are suitable for use in the present compositions. The polymeric dispersant may comprise a random polymer or a structured polymer, for example a block copolymer and/or branched polymer, or mixtures thereof, and the dispersant polymer may be anionic, cationic or nonionic in nature. Suitably, polymers having both hydrophilic sections for aqueous compatibility and hydrophobic sections for interaction with the pigment are preferred.
A further component of the aqueous ink jet compositions is the humectant. Humectants for use in ink jet ink compositions are known in the art and are suitable for use herein. Examples include, but are not limited to, alcohols, for example, glycols such as 2,2xe2x80x2-thiodiethanol, glycerol, 1,3-propanediol, 1,5-pentanediol, polyethylene glycol, ethylene glycol, diethylene glycol, propylene glycol and tetraethylene glycol; pyrrolidones such as 2-pyrrolidone; N-methyl-2-pyrrolidone; N-methyl-2-oxazolidinone; and monoalcohols such as n-propanol and iso-propanol. Preferably the humectants are selected from the group consisting of 2,2xe2x80x2-thiodiethanol, glycerol, 1,3-propanediol, 1,5-pentanediol, polyethylene glycol, ethylene glycol, diethylene glycol, propylene glycol, tetraethylene glycol, 2-pyrrolidone, n-propanol and mixtures thereof. In a preferred embodiment, the humectant comprises a mixture of alcohols. In a further preferred embodiment, the humectant comprises a mixture of 2,2xe2x80x2-thiodiethanol and a glycol such as a polyalkylene glycol.
The latex binder is employed in the ink jet ink compositions in order to improve the water-fastness of a printed recording medium, particularly as measured by wet-rub resistance. The latex binder is particularly advantageous in that it improves the wet-rub resistance while maintaining good print quality and good optical density, particularly after heating printed images. Additionally, the latex binder resists film formation on the nozzle plate and allows the ink compositions to properly eject from the nozzles during the printing operation. Accordingly, clogging of the nozzles by the ink composition is prevented.
The latex binder is formed by emulsion polymerization of selected components. Particularly, a combination of first and second monomers is employed in combination with a charge stabilizing agent, an emulsifier and an initiator. The first monomer is typically regarded as a harder monomer, i.e., a monomer having a higher glass transition temperature Tg, while the second monomer is conventionally considered a softer monomer, i.e., a monomer having a relatively lower glass transition temperature Tg. Reference within the present disclosure to the glass transition temperature of a monomer refers more specifically to the glass transition temperature of a homopolymer formed from the particular monomer. More specifically, the latex binders are formed from first monomer having a glass transition temperature Tg greater than about 70xc2x0 C. and comprising styrene, substituted styrene, methyl methacrylate or a mixture thereof. Substituted styrenes include alkyl-substituted styrenes, halogen-substituted styrenes and the like. In a preferred embodiment, the first monomer, or mixture thereof, has an average glass transition temperature Tg of about 100xc2x0 C., or greater. The second monomer has a glass transition temperature Tg less than about 0xc2x0 C. and comprises at least one C2-C10 alkyl acrylate. Examples of suitable alkyl acrylates include, but are not limited to, ethyl acrylate, propyl acrylate, butyl acrylate, octyl acrylate, ethylhexyl acrylate and the like. Preferably, the second monomer has a glass transition temperature Tg less than about xe2x88x9225xc2x0 C., and more preferably less than about xe2x88x9250xc2x0 C. The ratio of the first and second monomers may be varied so that the latex binder has a glass transition temperature Tg in the range of from about 0xc2x0 C. to 70xc2x0 C. and depending on additional desired properties for the latex binder. In a preferred embodiment, however, the latex binder is formed from at least about 50 weight percent of the first monomer, based on the weight of the first and second monomers, and more preferably comprises at least about 75 weight percent of the first monomer, based on the weight of the first and second monomers.
The reaction medium for preparing the latex binder employs both a charge stabilizing agent and an emulsifier in order to obtain the desired particle size. Particularly, the latex binder has an average particle size as measured by a Honeywell UPA 150 light scattering instrument of from about 150 nm to about 350 nm, and more preferably from about 200 nm to about 300 nm, and most preferably from about 250 nm to about 280 nm. Various charge stabilizing agents are known in the art and are suitable for use in preparing the latex binder of the present compositions. In a preferred embodiment, the charge stabilizing agent comprises methacrylic acid, acrylic acid, and/or a salt thereof. Sodium salts of methacrylic acid and/or acrylic acid are particularly preferred. The charge stabilizing agent may be employed in conventional amounts, and preferably in an amount of from about 0.1% to about 5% by weight of the emulsion polymerization components. Various emulsion polymerization emulsifiers are also known in the art and suitable for use in the present methods. However, in a preferred embodiment, the emulsifier comprises a fatty acid ether sulfate, and more preferably comprises a lauryl ether sulfate. Suitably, the emulsifier may be employed in conventional amounts and preferably in an amount of from about 0.1% to about 5% by weight of the emulsion polymerization components. Lauryl ether sulfate is preferred for obtaining inks which require low printer maintenance and provide good printer reliability.
The emulsifier is employed not only to obtain the desired particle size of the latex binder but further to obtain the desired surface tension of the latex binder in the range of from about 40 dynes/cm to about 60 dynes/cm. More preferably, the latex binder has a surface tension of from about 45 dynes/cm to about 55 dynes/cm, and even more preferably has a surface tension of about 50 dynes/cm.
The emulsion polymerization is conducted in accordance with conventional polymerization techniques, for example in a semi-batch process. The latex is synthesized by free radical initiated polymerization, and any free radical initiator known in the art may be employed. Preferably, the initiator comprises a per compound such as a persulfate, peroxygen, or the like. Persulfate initiators such as ammonium persulfate are particularly preferred. The initiator may be employed in conventional amounts and suitably is employed in an amount of from about 0.01 to about 5 weight percent, based on the weight of the emulsion polymerization components.
The aqueous ink jet ink compositions according to the present invention may employ the pigment, humectant, dispersant and latex binder in amounts suitable for obtaining desired print properties. In preferred embodiments, the aqueous compositions comprise, by weight, from about 1% to about 20% pigment, from about 5% to about 50% humectant, from about 0.01% to about 10% dispersant, and from about 1% to about 20% latex binder. More preferably, the compositions comprise, by weight, from about 1% to about 10% pigment, from about 10% to about 30% humectant, from about 0.1% to about 5% dispersant, and from about 1% to about 10% latex binder. Even more preferred are compositions comprising, by weight, from about 1% to about 5% pigment, from about 15% to about 25% humectant, from about 0.1% to about 4% dispersant, and from about 2% to about 5% latex binder.
The ink compositions may further include conventional additives known in the art. For example, the compositions may comprise one or more biocides to allow long term stability. Suitable biocides include benz-isothiazolin-one, methyl-isothiazolin-one, chloro-methyl-isothiazolin-one, sodium dihydroacetate, sodium sorbate, sodium 2-pyridinethiol-1-oxide, sodium benzoate and sodium pentachlorophenol. Examples of commercially available biocides are Zolidine(trademark), Proxel(trademark), Givguard(trademark), Canguard 327(trademark) and Kathon(copyright) PFM. The compositions may further include fungicides, bactericides, penetrants, surfactants, anti-kogation agents, anti-curling agents and/or buffers, various examples of which are known in the art. The inkjet ink compositions suitably have a pH of from about 8.0 to about 8.5.
The aqueous ink jet ink compositions may be prepared in accordance with conventional processing techniques. Typically, the pigment is combined with the dispersant to provide a pigment dispersion which is then combined with additional components of the compositions. The compositions may be employed in ink jet printing methods in a conventional manner, wherein a droplet of the ink composition is ejected through a printhead nozzle in response to an electrical signal and onto a surface of a paper recording medium. In one embodiment, a printed image is heated to improve its wet rub resistance. For example, a recording medium may be passed through a heating zone at a temperature greater than about 70xc2x0 C., preferably greater than about 100xc2x0 C., to improve the wet rub resistance of printed images thereon.
In one embodiment of ink jet printing methods according to the present invention, a droplet of an aqueous ink jet ink composition is ejected through a printhead nozzle and onto a surface of a paper recording medium, after which the paper recording medium with the aqueous ink jet ink composition on a surface thereof is passed through a fuser system at a temperature greater than about 100xc2x0 C. for about 5 to about 100 seconds. In this embodiment, the aqueous ink jet ink composition preferably comprises, by weight, from about 0.8% to about 20% pigment, from about 5% to about 50% humectant, from about 0.01% to about 10% dispersant, and from about 1% to about 20% latex binder. The latex binder has a glass transition temperature Tg of from about 0xc2x0 C. to about 70xc2x0 C. and a particle size of from about 150 nm to about 350 nm. By passing the printed recording medium through the fuser system, the wet-rub resistance of an ink composition is substantially improved while maintaining good print quality and optical stability. In a preferred embodiment, the fuser system is operated at a temperature greater than about 100xc2x0 C. and the paper recording medium passes through the fuser system for about 20 to about 60 seconds.
This method may be employed with various types of paper recording mediums, including plain papers such as copy paper, report paper and bond paper, as well as pretreated papers such as coated and glossy papers.
In additionally preferred embodiments of these methods, pressure is applied to the paper recording medium in the fuser system. For example, pressure may be applied to one side of the recording medium or, more preferably, to both sides of the recording medium. Suitable pressures applied to the paper recording medium in the fuser system are in the range of from about 15 psi to about 30 psi, and more preferably from about 18 psi to about 26 psi.
The ink jet ink compositions and ink jet printing methods according to the present invention provide printed recording mediums which exhibit improved wet-rub resistance. The improved wet-rub resistance is obtained while maintaining good print quality and optical density and without causing printer maintenance problems or printer failure owing to clogged nozzles and the like.